In the past, there have been hot-wire, air-mass flowmeters described for use in motor vehicles. U.S. Pat. No. 4,373,383 discloses such a system. According to U.S. Pat. No. 4,373,383, a resistor in the form of a hot-wire, air-mass flowmeter is used for measuring purposes in a motor vehicle. The hot wire is burned free of dirt particles in a free-burning operation which takes place after every time an internal-combustion engine is shut off. This is accomplished by a predetermined unbalancing of a bridge circuit which contains the resistor in a bridge arm.
In principle, this recurrent free-burning brings about good results. In particular, the drift in characteristics caused by the contamination of the resistor (which is generally made of platinum wire) is eliminated after each shut-off operation of the internal-combustion engine. However, along with every free-burning operation, parts of the wire material of the resistor are also vaporized, so that after a large number of free-burning operations, there is a disadvantageous distortion of characteristics.
In the past, there have been other systems that are not hot wire systems that have the ability to prevent contamination of an element used in a measuring device. U.S. Pat. No. 4,125,093 discloses a flowmeter which is based on a semiconductor and can be installed in automotive fuel injection systems to measure air mass and fuel quantity. To prevent the sensor from being contaminated, U.S. Pat. No. 4,125,093 proposes covering the semiconductor sensor with a passivation layer of silicon nitride and possibly with gold plating.
Prior systems also have used integration and comparing steps in the context of free-burning operations. Japanese Patent Nos. 60 125 751 and 63 184 018 disclose methods of integrating or summing up the suctioned air mass and then comparing the integration or cumulative value with a limiting value for the free-burning operation. When the integration or cumulative value exceeds the limiting value, a free-burning operation is initiated.